void M5 <T>(C16 <T, int> c) where T : class
{
// Viable callables: {C16,C17}.M1()
c.M2(() => default(T));
}
void M4 <T>(C16 <T, int> c) where T : struct
{
// Viable callable: C16.M2() [also reports C17.M2(); false positive]
c.M2(() => default(T));
}